In a way I was thinking it could be a ram problem... Vista uses about 2 times more memory than XP so if he does not have enough memory in his system vista could interfere with the games performance. After all from personal experience CS:S should run at almost the same speed on Vista as it does in XP.
Besides I like the new layout of Vista and the removal of direct draw on the desktop makes it too good to go back to XP now. People ph34r change, and that’s very obvious with the way current events are panning out.
you could try overclocking that 2500+ to a 3200+ by a simple FSB increase, that might help a fair bit (make sure you dont increase the mem timings at the same time). Also your graphics card is fine for vista as I was beta testing it on a X800XT PE with a SB Audigy 2 ZS Platinum Pro and I got no problems what so ever.